Air compressor powered by differential gas pressure

ABSTRACT

A gas-driven air compressor system and a method for using it. No electricity, lubrication or cooling water is required. A high pressure gas used to drive the compressor may be recovered at a pressure high enough to be retain its economic value. In one preferred embodiment, the air compressor includes a gas-driven drive cylinder and an air-driven boost cylinder, interconnected by reciprocating drive and boost pistons. The drive piston supplies force to power the boost piston, which pulls in atmospheric air and discharges it at a higher pressure for use by pneumatically-operated controls and equipment. A four-way valve operating on differential gas pressure may be used to automatically actuate the reciprocating piston. Using the present invention, fugitive gas emissions, such as normally occur when using a separator to remove oil and water from a wellhead gas stream, may be avoided.

BACKGROUND OF THE INVENTION

The present invention relates to gas-driven air compressors. More specifically, the present invention relates to methods and systems for compressing air using energy available in a high pressure gas stream, while exhausting the drive gas to a lower pressure gas stream and still retaining the commercial value of the gas.

There is a need for using gas-driven air compressors in applications such as oil and gas wells. In many cases, particularly in remote, rural areas, no electricity may be available, and all equipment may be run off of natural gas. Available gas pressures can be substantial (e.g., 200-1000 psi). In a gas well, for example, gas from the well enters a separator to remove oil and water. The gas is filtered and transported to a “sales line,” which collects gas and transports it to a natural gas processing facility. Sales line pressure may be in the 100-250 psi range. The controls which operate the separator are run off of natural gas, which is vented to the atmosphere. It would be advantageous to develop a system for eliminating the fugitive gas emissions from the separator controls by operating the controls on compressed air. If the air compressor was driven by the differential pressure that exists between the well head and the sales line, there would be no energy cost. If the discharge from the gas drive is fed into the sales line, there would be no pollutant emissions associated with the gas-driven air compressor and no loss of revenue.

These fugitive emissions of volatile organic compounds are a safety and environmental hazard. In Colorado, for example, environmental standards were implemented in December of 2006 in an effort to reduce volatile organic compound emissions which create ozone and negatively affect air quality. These standards were made more stringent after May of 2008 to help reduce the high levels of ozone concentrations in the area and to keep Colorado in compliance with national air standards.

By way of backround, air compressors typically include a drive system which provides the energy required to operate the air compression system, and an air compression system which elevates air pressure. The drive systems may include: a crankcase driven by an electric motor or an engine; a turbine drive; and a hydraulic piston driven by an electric motor or an engine.

As further backround, the air compression system may include: a reciprocating piston (providing moderate compression ratios and flowrates, suitability for high operating pressures, low to moderate cost, a compact design, rod seal leakage and vibration, and a moderate operating life for the seals, especially non-lubricated seals); a turbine (providing high flowrates, low vibration, a long operating life, suitability for high pressures, low compression ratios, high cost, shaft seal leakage, and a large size); a diaphragm (providing high compression ratios, no seal leakage, suitability for high pressures, very low flow, high cost, vibration, and a low operating life); a bellows (providing no seal leakage, moderate cost, low flow, low compression ratios, vibration, and a lack of suitability for high pressures); a rotary vane (providing high flowrates, low cost, low compression ratios, a lack of suitability for high pressures, and a low operating life); a fan (providing high flowrates, low cost, very low compression ratios, and a lack of suitability for high pressures); a “roots type” blower (providing high flowrates, moderate cost, long life, low compression ratios, shaft leakage, and a lack of suitability for high pressures); and a rotary screw (providing high flowrates, moderate cost, long life, moderate compression ratios, shaft leakage, and a lack of suitability for high pressures).

With low flowrates and moderate compression ratios, which are the focus of the preferred embodiment described below, the most practical air compression system utilizes a reciprocating piston. Most existing piston air compressors utilize crankcase drives. For many applications, such as pneumatic controls on natural gas or oil production equipment, electricity is either not available, or not available in sufficient quantities to drive an air compressor. Gas or oil-fueled engine compressor drives are impractical due to first cost, maintenance costs, fuel costs and pollutant emissions. Accordingly, compact air compressors powered by a gas-driven reciprocating piston may be a good engineering fit in such applications which have high pressure gas (instead of compressed air) available to drive the booster.

Gas-driven compressors, also called gas boosters, booster compressors and air amplifiers, that utilize compressed gas (or compressed air) as the motive force to boost air pressure, are known. They are typically used to boost shop air pressure for applications which require higher pressures than the utility air pressure available. Gas-driven compressors have various advantages: the pressure boost in such devices can be as low as 5 psi or as high as thousands of psi; they require no electricity, cooling water or lubrication; and they are explosion-proof, compact, easy to install and economical. Such advantages may be important in applications located in remote areas where electricity may not be available (e.g., oil and gas wells). Gas-driven compressors are available, for example, from Midwest Pressure Systems, Inc. of Bensenville, Ill.

With existing gas-driven compressors, the air compression section includes a single-acting or double-acting cylinder with inlet and discharge check valves for each pumping chamber. There are variations in check valve, piston and rod seal designs, and materials, but all of the existing systems are similar in engineering design.

The drive section of the boosters may have several variations, but generally consist of a four-way valve which causes the drive piston to reciprocate automatically. The differences are in the manner used to actuate the valve:

1. Mechanical actuation causes the four-way valve to shift as a result of the drive piston mechanically moving the valve element at the end of stroke.

2. Pilot shifting actuates the four-way valve through a small amount of pressurized air or gas which forces a piston attached to the valve to move, causing the valve to shift. There are three versions of this design. A first version uses a four-way valve with a double-pilot design which receives a pilot signal at each end of the valve. With this first version, pilot valves are triggered by the piston at the end of each stroke. Each pilot valve sends a pilot air or gas signal to the four-way valve, causing it to shift. After the four-way valve shifts, the pilot air or gas is vented. The second version uses the same two pilot valves, but one valve sends a pilot signal to the pilot side of a single-pilot, spring-return, four-way valve. The pilot air or gas shifts the four-way valve against the spring and remains trapped in the pilot section until the other pilot valve is tripped, venting the air or gas in the pilot section. With this second version, the spring then shifts the four-way valve back to the original position. The third version is similar to the second version. Pilot air or gas actuates a larger pilot piston on one side of the four-way valve and holds it in place. The piston on the other side of the four-way valve is smaller, and is always charged with supply air or gas. When pilot air or gas is vented from the first piston, the smaller piston shifts the four-way valve back to its original position.

3. Existing gas-driven compressor designs vent the drive gas to air atmosphere. The pilot air or gas also vents to atmosphere. The drive force is determined by the pressure of the drive air or gas above atmospheric pressure. The flow capability is a function of this drive force as well as the amount of drive air or gas that is available. Typically, the maximum pressure rating of gas booster drive systems is 10 barg or 150 psig, which encompasses the shop air pressure available in most industrial applications.

Rod seal design and materials, piston seal design and materials, and structural materials vary in the pneumatic drive section, but the various models are similar in engineering design.

Accordingly, there is a need for a system and method for eliminating the fugitive gas emissions from gas-operated controls and equipment (such as but not limited to well separators) by operating the gas-operated controls and equipment on compressed air. It would be advantageous to drive the air compressor using differential gas pressure (such as but not limited to that differential gas pressure that exists between a well head and the sales line), in which case there would be no additional energy cost. It would also be advantageous to feed the discharge from the gas drive into the sales line, so that there would be no pollutant emissions associated with the gas-driven air compressor, and no loss of revenue.

Definition of Claim Terms

The terms used in the claims of the patent as filed are intended to have their broadest meaning consistent with the requirements of law. Where alternative meanings are possible, the broadest meaning is intended. All words used in the claims are intended to be used in the normal, customary usage of grammar and the English language.

“Atmospheric pressure” means the pressure exerted by the atmosphere. This pressure is 14.7 psia (absolute pressure) at sea level or 0 psig (gauge pressure). Atmospheric pressure falls as elevation increases. For example, atmospheric pressure in Denver, Colorado at 5280 feet elevation is approximately 12.1 psia asolute pressure and 0 psig gauge pressure.

“Compression ratio” means the ratio of the increased pressure of the air over atmospheric pressure.

SUMMARY OF THE INVENTION

The objects mentioned above, as well as other objects, are solved by the present invention, which overcomes disadvantages of prior gas delivery and air compression systems and methods, while providing new advantages not believed associated with such systems and methods.

In a preferred embodiment of the invention, a gas-driven air compressor is provided, and includes a drive cylinder and an air compression cylinder interconnected by reciprocating drive and compressor pistons. Initially, the drive cylinder may be filled with drive gas at a first gas pressure, moving the drive gas piston through a full stroke based on the length of the cylinder. At the end of the stroke, drive gas may be vented at a second gas pressure which is lower than the first gas pressure. The drive gas piston is connected to the air compressor piston, so that they work in tandem. During a piston stroke, one side of the compressor cylinder may be charged through an inlet check valve with air at atmospheric pressure. The other side of the compressor cylinder may be used to compress air to an elevated pressure, and discharge the air through a check valve to a storage tank or to pneumatically-operated equipment.

In a particularly preferred embodiment, a four-way valve operating on differential gas pressure may be used to actuate the reciprocating pistons. The four-way valve may be actuated in various ways. For example, it may be actuated using gas pilot pressure, or a mechanical actuation, applied on each side of the valve. As another example, the four-way valve may be actuated by pilot pressure applied on one side of the valve; when this pressure is vented, a spring may be used to actuate the other side of the valve. As a further example, the four-way valve may be actuated by pilot pressure applied to a valve piston acting on one side of the valve; when this pilot pressure is vented, supply pressure acting on a smaller valve piston on the other side of the valve may be used to actuate the valve. In each case, return of the valve may be actuated by venting the pilot pressure to a low gas pressure line.

The gas pressure booster may be operated without the need for electricity, lubrication or cooling water.

In a particularly preferred embodiment, the first gas stream may originate from a well head, and the compressed air supplied from the air compressor may be used to operate pneumatic controls for (e.g.) a separator used to remove oil and water from the first gas stream.

In another embodiment of the invention, a method is disclosed for using a gas-driven air compressor having a gas drive cylinder and an air compression cylinder interconnected by reciprocating gas drive and air compression pistons. Using the first gas stream on one side of the drive gas cylinder, the drive piston is moved within the drive cylinder, exhausting a second gas stream on the other side of the drive gas cylinder at a lower pressure. The air compression piston moves under the force supplied by the drive piston, inducing air at atmospheric pressure to flow into one side of the air compression cylinder, thereby boosting air in the other side of the air compression cylinder to a higher pressure than atmospheric pressure. In one method example, the first gas stream can originate from a well head, the second gas stream may be fed to the “sales line,” and compressed air may be supplied from the air compressor and used to operate the pneumatic controls of a separator for removing oil and water from the first gas stream. The four-way valve may be used in the method claim in various ways in a similar manner to how it is used in the system claim.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features which are characteristic of the invention are set forth in the appended claims. The invention itself, however, together with further objects and attendant advantages thereof, can be better understood by reference to the following description taken in connection with the accompanying drawings, in which:

FIGS. 1-4 are progressive, illustrative schematic views showing air compressor stages 1-4 (corresponding to FIGS. 1-4) for a gas-driven air compressor system according to a preferred embodiment of the present invention.

The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. In the drawings, like reference numerals designate corresponding parts throughout the several views.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Set forth below is a description of what are believed to be the preferred embodiments and/or best examples of the invention claimed. Future and present alternatives and modifications to this preferred embodiment are contemplated. Any alternatives or modifications which make insubstantial changes in function, in purpose, in structure, or in result are intended to be covered by the claims of this patent.

FIGS. 1-4 show progressive schematic views of a preferred embodiment of a gas-driven air compressor system of the present invention, generally referred to by the reference numeral 10. Referring to the figures, air compressor system 10 includes a high pressure, gas-driven drive cylinder 20 and an air compression cylinder 30. High pressure gas emanates from a source 60, while low pressure gas flows to sink 80. Drive cylinder 20 and air compression cylinder 30 share a common piston rod 40, so that drive piston 23 and air compressor piston 33 move in tandem. Four-way valve 50 is in fluid communication with the front and back sides of drive cylinder 20, as well as with high pressure gas source 60 and low pressure gas sink 80, and the four-way valve moves in the manner described below.

Referring now to FIG. 1, stage 1 of the air compressor system's operation will be described. The direction of the stroke of piston rod 40 is shown by the arrow located above drive piston 23 (i.e., the piston rod is moving right-to-left). In the middle of a forward stroke, high pressure gas from source 60 (e.g., from the wellhead and regulated to a pressure level above the sales line pressure, which may be in the, e.g., 100-750 psi range in this application) flows through line 75, which fills chamber 21 and pushes drive piston 23, piston rod 40 and air compression piston 33 in the direction of stroke shown. Air compression piston 33 induces a vacuum in chamber 31, closing check valve 36, and causing atmospheric air from line 70 to flow through check valve 34 and fill chamber 31. Concurrently, air compression piston 33 pushes air from chamber 32, which closes check valve 35 and exits through check valve 37 at elevated pressure through line 72. Low pressure gas at the sink pressure also exits chamber 22 of drive cylinder 20 through line 74, moving through 4-way valve 50 and to low pressure sink 80.

Referring now to FIG. 2, the second stage occurs when drive piston 22 reaches the end of its forward stroke. At this point, all of the air has been pushed out of the chamber 32 of the air compression cylinder 30 through check valve 37 and line 72, and chamber 31 of the air compression cylinder is fully charged with air at near-atmospheric pressure. Drive cylinder chamber 21 is fully charged with high-pressure gas. Now, the drive piston triggers a pilot valve (not shown), which shifts four-way valve 50, leading to the third stage shown in FIG. 3.

Referring now to FIG. 3, in the third stage of the air compressor system's operation, 4-way valve 50 shifts to the right. After this shift occurs, high pressure gas from the chamber 21 of the drive cylinder flows through line 75, the four-way valve, line 76, and into low pressure sink 80. Air from chamber 31 of the air compression cylinder is at near-atmospheric pressure. High pressure gas starts flowing from source 60, through the 4-way valve, through line 74, and into chamber 22 of the drive cylinder, initiating the reverse stroke of the drive piston 23, piston rod 40 and air compression piston 32.

Referring now to FIG. 4, which shows the fourth and final stage of the air compressor system's operation, in the middle of a reverse stroke. High pressure gas from line 74 pushes drive piston 23, piston rod 40 and air compression piston 33 left-to-right in the direction of the arrow. The remaining gas in chamber 21 is pushed out through line 75, four-way valve 50 and line 76 to low pressure sink 80. Atmospheric air is sucked into chamber 32 of air compression cylinder 30 through line 82 and check valve 35. Low pressure in chamber 32 closes check valve 37. Concurrently, compressed air exits the back-side chamber 31 of air compression cylinder 30, through check valve 36 and line 83. High pressure in chamber 31 causes check valve 34 to close. When drive piston 23 reaches the end of the reverse stroke, it triggers a pilot valve (not shown) which again switches the four-way valve 50, iniating a new forward stroke for the piston rod.

Those of ordinary skill in the art will appreciate that the gas-driven air compressor of the present invention may be advantageously employed to supply compressed air from an available high pressure gas source, and that it may be used in a variety of devices and systems, including but not limited to oil or gas wells.

The above description is not intended to limit the meaning of the words used in the following claims that define the invention. Persons of ordinary skill in the art will understand that a variety of other designs still falling within the scope of the following claims may be envisioned and used. It is contemplated that future modifications in structure, function, or result will exist that are not substantial changes and that all such insubstantial changes in what is claimed are intended to be covered by the claims. 

I claim:
 1. A gas-driven air compressor, comprising: a gas drive cylinder and an air compression cylinder interconnected by reciprocating gas drive and air compression pistons; the drive piston supplying force powered by a first gas stream at a first pressure within the drive cylinder which exhausts to a second gas stream at a second pressure lower than the first pressure; and the air compression piston moving under the force supplied by the drive piston, which movement induces air at atmospheric pressure to flow into the air compression cylinder and boosts the pressure of the air in the air compression cylinder to a higher pressure than atmospheric pressure.
 2. The air compressor of claim 1, further comprising a four-way valve actuating the reciprocating piston, the four-way valve operating on differential gas pressure.
 3. The air compressor of claim 2, wherein movement of the four-way valve is actuated by gas pilot pressure applied on each side of the valve.
 4. The air compressor of claim 2, wherein movement of the four-way valve is actuated by gas pilot pressure applied on one side of the valve.
 5. The air compressor of claim 4, wherein venting of the gas pilot pressure results in a spring actuating a side opposing the one side of the valve.
 6. The air compressor of claim 2, wherein movement of the four-way valve is actuated by gas pilot pressure applied to a first valve piston on one side of the valve.
 7. The air compressor of claim 6, wherein venting of the gas pilot pressure results in supply pressure actuating the valve by acting on a second valve piston on a side opposing the one side of the valve, wherein the second valve piston is smaller than the first valve piston.
 8. The air compressor of claim 3, wherein a return movement of the valve is actuated by venting the gas pilot pressure to a low pressure gas line.
 9. The air compressor of claim 4, wherein a return movement of the valve is actuated by venting the gas pilot pressure to a low gas pressure line.
 10. The air compressor of claim 6, wherein a return movement of the valve is actuated by venting the gas pilot pressure to a low gas pressure line.
 11. The air compressor of claim 1, further comprising a mechanically-actuated four-way valve actuating the reciprocating gas drive piston.
 12. The air compressor of claim 1, wherein the air compressor operates without the need for electricity, lubrication or cooling water.
 13. The air compressor of claim 1, wherein the air compression piston pulls air at atmospheric pressure into the air compression cylinder through an inlet check valve on a forward stroke of the air compression piston, and pushes air at an elevated pressure above atmospheric pressure out of the air compression cylinder through a discharge check valve on a reverse stroke of the air compression piston.
 14. The air compressor of claim 1, wherein the air compression piston pulls air at atmospheric pressure into the air compression cylinder through an inlet check valve on one side of the cylinder, while simultaneously pushing air at an elevated pressure above atmostpheric pressure out of the air compression cylinder on the other side of the piston through a discharge check valve on a forward stroke of the air compression piston, and repeats the process on a reverse stroke of the air compression piston.
 15. The air compressor of claim 1, wherein the first gas stream originates from a well head, and wherein the compressed air supplied from the air compressor is used to operate controls of a separator used to remove oil and water from the first gas stream.
 16. A method for using a gas-driven air compressor having a gas drive cylinder and an air compression cylinder interconnected by reciprocating gas drive and air compression pistons, comprising the steps of moving the drive piston using a first gas stream at a first pressure within the drive cylinder which exhausts to a second gas stream at a second pressure lower than the first pressure; and the air compression piston moving under the force supplied by the drive piston, which movement induces air at atmospheric pressure to flow into the air compression cylinder, thereby boosting air within the air compression cylinder to a higher pressure than atmospheric pressure.
 17. The method of claim 16, wherein the first gas stream originates from a well head, and further comprising the step of using the compressed air supplied from the air compressor to operate controls of a separator used to remove oil and water from the first gas stream.
 18. The method of claim 16, wherein a four-way valve is provided in fluid communication with the reciprocating gas drive piston, and further comprising the step of actuating movement of the four-way valve using differential gas pressure between the first and second gas streams.
 19. The method of claim 18, further comprising the step of actuating movement of the four-way valve using gas pilot pressure applied on at least one side of the valve.
 20. The method of claim 16, further comprising the step of the air compression piston pulling air at atmospheric pressure into the air compression cylinder through an inlet check valve on a forward stroke of the air compression piston, and the air compression piston pushing air at an elevated pressure above atmospheric pressure out of the air compression cylinder through a discharge check valve on a reverse stroke of the air compression piston. 